When the Scottish owner of Castle Keep died, the ancient castle went to his Canadian great-nephew, Robert Volnik. There was no way the Volniks could keep the castle, so it was sold to an Edinburgh lawyer, Mr. Maconochie. Two years later, Emily Volnik and her younger brother Jessup return to the castle. To their delight, the Boggart, a mischievous shape-shifting spirit who has lived in the castle for centuries, playing tricks on the owners, is still there.

At Jessup's urging, Mr. Mac takes them on a camping trip to Loch Ness, where a new expedition with advanced underwater equipment is planning another search for the Loch Ness Monster. The Boggart comes along and, on their first night there, he rediscovers Nessie, a cousin who has forgotten how to change shape and who, though a boggart, remains in the prehistoric monster form he long ago adopted. Excitement mounts as the expedition sights Nessie, and, in a desperate struggle, the children and Mr. Mac strive to help the Boggart restore Nessie to his rightful boggart form and escape. Beautifully imagined and beautifully written, this is a stunning companion to Susan Cooper's earlier book, The Boggart.